8. Nerves
The brachial plexus is a complex neural network formed by lower cervical and upper thoracic ventral nerve roots which supplies motor and sensory innervation to the upper limb and pectoral girdle. It is located in the neck extending into the axilla posterior to the clavicle. Summary * origin: ventral rami of C5 to T1 * course: ** emerges between anterior and middle scalenes as 3 trunks ** through the posterior triangle of neck behind the clavicle as 3 divisions ** closely associated with the axillary artery in the axilla as 3 cords ** terminate as the 5 major nerves * major (terminal) branches ** median nerve ** ulnar nerve ** radial nerve ** axillary nerve ** musculocutaneous nerve * motor supply: upper limb and pectoral girdle * sensory supply: upper limb and part of the upper thorax Gross anatomy The brachial plexus consists of roots, trunks, divisions, cords and terminal branches as it travels from proximal to distal upper limb. This mnemonic describes the order of these subdivisions. Roots Roots are formed between the scalenus anterior and scalenus medius muscles by the anterior rami of C5-C8 and T1 nerve roots. Three branches arise from the roots directly: * dorsal scapular nerve: C5 root and supplies the rhomboids * subclavian nerve: C5 and C6 roots to supply subclavius * long thoracic nerve: C5-C7 and supplies serratus anterior In addition, the scaleni and longus colli muscles are supplied by multiple variable unnamed muscular branches that arise from all or some of the roots. It is important to remember that although not part of the brachial plexus: * the dorsal rami course posteriorly into the spinal extensors (erector spinae) and do not contribute to the brachial plexus. * the ventral rami of C4 and C5 also contribute to the phrenic nerve * the ventral ramus of T1 also contributes to the first intercostal nerve Trunks Trunks form from the roots as they pass between scalenus anterior and medius muscles, then go on to traverse the posterior triangle: * C5 and C6 roots combine to form the upper or superior trunk * C7 root makes up the middle trunk * C8 and T1 roots combine to form the lower or inferior trunk One branch arises from the trunks: * suprascapular nerve: arises from the upper trunk and supplies the supraspinatus and infraspinatus muscles, and sensation to the glenohumeral and acromioclavicular joints Divisions Each trunk divides to form an anterior and posterior division posterior to the mid clavicle. In general anterior divisions supply muscles of the anterior compartments (flexors) whereas the posterior divisions supply muscles of the posterior compartments (extensors). No branches arise from the divisions. Cords The divisions then combine to form cords, which are named for their relation to the second part of the axillary artery: * lateral cord is formed by the union of the upper two anterior divisions at the lateral border of the first rib * medial cord is a continuation of the lower anterior division * posterior cord consists of the united three posterior divisions The prevertebral fascia of the neck extends down to sheath the axillary artery and cords. It is into this axillary sheath that local anaesthetic is injected when performing a brachial plexus block 3. Peripheral nerves Various peripheral nerves then branch from these cords. Branches from the lateral cord are: * lateral pectoral nerve * musculocutaneous nerve * lateral root of the median nerve Branches from the posterior cord are (STARS): * upper subscapular nerve * thoracodorsal nerve * axillary nerve * radial nerve * lower subscapular nerve Branches from the medial cord are: * medial pectoral nerve * medial cutaneous nerve of the arm * medial cutaneous nerve of the forearm * medial root of the median nerve * ulnar nerve Variant anatomy Approximately 50% of the population show significant variation in formation of the brachial plexus: * pre-fixed brachial plexus: contribution from C4 ventral ramus, with overall contribution from C4-C8 (i.e. the roots are moved up one vertebral level) * post-fixed brachial plexus: contribution from T2 ventral ramus, with overall contribution from C6-T2 (i.e. the roots are moved down one vertebral level) ** the inferior trunk is at risk of compression by the first rib * contributions of C4 or T2 ventral rami (6 or 7 roots) * trunk, divisions or cord may be absent * variations in branching patterns of the cords have also been reported 1